Filter



Nov. 25, 1941.

Ejl.. OLIVER ETAL FILTER Filed Nov. 30, 1959 3 Sheets-Sheet l Nov. 25, 19.41. E. L. oLlvE-R ET AL v 2,263,852

FILTER Filed-Nov. 50,]1939 s sheets-sheet 2 Patented Nov. 25, 19.41

FILTERl Edwin L. Oliver, Piedmont, Edward D. Flynn,

Oakland, and Orville B. Ackerly, Jr., Piedmont, Caiii'., assignors to Oliver United Filters Incorporated, Oakland, Calif., a corporation of Nevada Application November 30, 1939, Serial No. 306,914

claims.

This invention relates in general to continuous rotary drum iilters and more particularly to filters of this type capable of handling large volumes of liquor.

Filtration on any type 'of lter requires the use of a diierential ltering pressure; To discharge more readily the cake formed as a result of the iiltering operation, this differential pressure should be relieved. In the standard form of continuous filters now in use, the imposition of a differential iiltering pressure during the so-called pick-up or cake forming cycle and the release of this' pressure during the discharge cycle are effected by an automatic valve communicating through suitable piping with the various filtrate compartments of the lter. For certain iltration operations such as the washing of paper pulp on continuous rotary rum filters, this piping may be dispensed with by simply maintaining a differential iiltering pressure between the exterior and interior of the drum except at that portion of the drum from which the cake is being discharged.

One of the objects of this invention is the provisicn in the sectionalized drum of a continuous rotary drum filter, of peripherally aligned ported pockets depending from each oi the drum sections or compartments and arranged to register with a stationary shoe once during each revolution of the drum.

Another object of this invention is the provision in the drum of a rotary drum lter o! a stationary shoe arranged to cut off communication between the interior of the drum and successive iilter sections, and provided with a compartment for effecting a separation between the ltrate per se and the liquorused to -wash the cake deposited on the drum.

Another object of this invention is the provision in the sectionalized drum of a rotary drum filter, of a stationary shoe adapted to concurrently cut off communication between the interior of the drum and two or more of its sections or compartments, and provided with a by-pass by which communication may be established between two or more of the compartments in registration with the shoe. f

A further object of this invention is the provision in a rotary drum iilter of a cake dischargey mechanism external to the drum and by means of which a iiuid pressure may be created beneath the cake to lift it from the iilter medium.

A still further object of the invention is the provision of a novel means of so feeding pulp or slurry to the tank of a continuous iilter that a more uniform density` of the pulp within the tank is maintained.

Another object of the invention is the provision in a continuous iilter of a spray mechanism for subjecting the pulp carriedl on the drum of a rotary drum iilter to a spray wash.

Another object of this invention is the provision in a rotary drum lter ci a press x'oll located along the ascending side of the drum substantially at the liquor level for the purpose o f compacting the cake at this point to prevent it from sloughing off the drum.

The invention possesses other advantageous features, some of which with vthe foregoing will be set forth at length inthe following description where that form of the invention which has been selected for illustration inlthe drawings accompanying and forming a part of the present specification is outlined in full. Inl said drawings, one form of the invention is shown. but it l is to be understood that it is not limited to such form, since the invention as set forth in the claims may be embodied in a plurality of forms.

Referring'to the drawings: y Figure 1 yis a longitudinal, vertical midsection of a rotary drum filter embodying the objects of my invention and taken on the line I-I of Figure 6.

Figure 2 is an enlarged "section taken on the line 2-2 of Figure 1. l

Figure 3 is a side elevation of one of the stationary shoes used to close successively the ports of the filtrate compartments as shown in Figure 2.

Figure 4 is an end elevation of the shoe shown in Figure 3.

Figure 5 is a section taken on the line 5-5 of Figure 3.

Figure (iis an end elevation of the iilter shown in Figure 1 with a portion of the end of the tank broken away and with a.A portion of the drum in section, the better to illustrate its constructie and operation.

Figure 7 is a partial side elevation of the doctor used for discharging the cake from the iilter shown in Figures 1 and 6.

Figure 8 is a transverse section taken through the doctor shown in Figure 7.

Figure 9 is a section taken on the nline 9 9 of. Figure 7. v i

As shown in these various ilgures, the'objects of our invention are embodied in a rotary drum `iilter comprising a tank generally designated by the reference numeral Il and consisting of a semi-cylindrical shell Il closed at its ends by tank heads I3 and Il welded thereto. Extending downwardly from the bottom of the shell I2 is a anged drainage port I5, and extending outwardly from the shell I2 adjacent the rising side of the drum are a plurality of longitudinally spaced flanged intake ports I5 (Figures 1 and 6) 'I'he tank I I is carried by pedestals I1 and I8 formed integral with the tank heads and these pedestals support longitudinally spaced bearings I 9 and 2I for the reception of coaxial hollow trunnions 22 and 23.

The trunnion 22 passes through a stuiling box 24 associated with the tank head I3 and is secured at its inner end to a dummy drum head 25.- Secured to the trunnion 22 intermediate its ends is a drum head 28 welded or bolted to a drum 21. The dummy drum head 25 is likewise welded or otherwisesecured at its periphery to the drum 21.

In the same manner the hollow trunnion 23 passes through a stuiling box- 28 associated with the tank head I4 and is secured to a drum head 29 and toa dummy drum head 3I, and these two latter members are in turn secured to the drum 21.

Formed on the periphery of the drum 21 are a plurality of illtrate sections or compartments 32 dened by peripherally spaced longitudinally extending division strips 33. Disposed within each of the compartments so formed is a drainage `member and supporting screen consisting of a plurality. of peripherally spaced longitudinally extending strips 34. Over the structure so formed is wrapped a illter medium 35 which in the present instance is in the form of a woven wire cloth and is held in place by a plurality of longitudinally spaced Acme bands 38.

Welded around the inner surface of the drum 21 intermediate its ends are a pair of spaced annular rings 31 and 38, and welded to the edges of these rings is aband 39 which with the rings forms an annular trough circumscribed by the drum 21. The annular trough so formed is divided into a plurality of valve bodiesor pockets 4I by radial walls or partitions 42 extending inwardly as partial extensions of the division strips 33, and communication between each of the pockets 4I and the ltrate compartment im.- mediately above it is eiected by a port 43 formed in the drum 21. The annular rings 31 and 38 constituting the side walls of the pockets 4I are formed on their outer faces with spaced annular machined valve seats 44. Assuming that the drum 21 is designed to rotate clockwise as indicated in Figures 2 and 6, each of the pockets 4I communicates .with the cylindrical chamber dened by the drum. 21 and the dummy drum heads 25 and 3|, through a port 45 formed in the ring 31 adjacent the trailing end of the.pocket and through a port 45 formed in the ring 38 adjacent the leading end of the pocket. 'I'his construction therefore permits liquor to pass into the drum through the ports 45 on the rising side of the drum and through ports 45 on the descending side of the drum, taking full advantage of the action of gravity. Furthermore, by making the peripheral length of these ports relatively short, each port as it engages a cut-01T shoe to be vpresently described can be closed in a relatively short interval of time, thereby making possible a sharper separation of illtrate and wash water when that is desired.

Extending' through a bearing 5I associated with the outer end of the hollow trunnion 22 and through a stuillng box 52 .associated with the inner end of this trunnion is a hollow shaft 53. Bolted to the inner end of the hollow shaft 53 in peripheral alignment with the pockets 4I is a split collar 54 formed with a pair of radially extending anged spokes or brackets 55 and 55, and bolted to these brackets are extensions 51 and 58. The outer ends of these extensions are provided on one side with flanges 59 and on the other side with ilanges 5I. The rfaces of these flanges are co-planar with the valve seats 44 formed on the annular rings 31 and 38.

Bridging the left hand side of the extensions 51 and 58 as viewed in Figure l is an arcuate shoe consisting of three identical shoe sections 52 bolted together by end anges and secured to 4the extensions 51 and 58 by bolts 53 passing its descending side.

through the two end sections 62 and the anges 59. The inner surface of the shoe formed by these identical sections is finished on its inner face and is designed to close off each of the ports 45 once during each revolution of the drum. As shown in Figure 6, this shoe subtends an arc of about 150, the upper section 82 extending well to the left of the vertical center line of the drum and` the lower section 82 terminating about 45 below the horizontal center line of the drum on The shoe is made in sections of such dimensions that they can be introduced into the drum through the hollow trunnion 23 and for the'same reason all other parts associated with the shoe and not formed integral with the interior of the drum are either made in sections or are suillciently small to pass through this trunnion. Y

The ports 48 formed on the leading ends of the pockets 4I (Figures 1 and 2) successively register with or contact an arcuate wash water chamber 1I, an arcuate by-pass shoe section 12, and a plain arcuate shoe section 13 identical with the shoesections 52 (Figure 6). As shown in Figure 1, the wash water chamber 1I is bolted to the flange 5I of the extension 51 and in a similar manner the shoe section 13 is bolted to a similar flange formed on the extension 58 (Figure 6) The `by-pass shoe section 12 is bolted to the inner ends of the shoe section 13 and the wash water chamber 1I, thereby forming a continuous integral arcuate structure therewith. Tire external faces of the shoe sections are formed with ribs 14 for strength and with end ilanges 15 for bolting the sections together.

The by-pass shoe section 12, as shown in Figures 3, 4, and 5, is formed on its inner face with an arcuate channel or by-pass 15, and machined along the sides ofv this channel are block seats 11 designed to accommodate one or more bridging blocks 18. The channel 16 extends through the lower end of the section 12 but terminates at a point 19 just short of the upper end of the section so that communication between the channel and the wash water chamber 1I is prevented. 'I'he by-pass shoe section 12 is designed to be located just opposite that portion of the drum from which the cake is to be discharged and subtends about three of the ltrate compartments 32, as also do the Awash water chamber 1I and the othershoe sections. It will therefore be seen that the by-pass or channel 15 in bridging two or more of the ports 45 of the pockets 4I can be used to establish communicationwith a discharged filtrate compartment 32 and one or more succeeding compartments which are approaching their discharge position. This then serves as a means for equalizing the pressure bemitting an easier and more effective discharge of the cake carried thereon. The bridging blocks 18 can be used to control the volume of air passing through the by-pass and the number of filtrate compartments to be affected.

It will be noted that as shown in Figure 6, the shoe section 13 and its opposed shoe section 462 are below the level of the intake ports I6. The purpose of these shoe sections is therefore to keep the ports 45 and 46 closed between the dis-L charge point and the point of submergence so that a differential filtering pressure can be maintained between the exterior and interior of the drum.

The purpose of subjecting many pulps and slurries to filtration is to wash the pulp or slurry free of chemical solutions. `Paper pulps, for example, must be free of acids or alkalies before they can be used in the manufacture of paper. The economy of the process, however, demands that these chemicals be recovered for re-use, and since this is ordinarily done by evaporation, it is desirable to avoid dilution of the filtrate. In the filter disclosed herein the filtrate per se passes directly into the interior of the drum, whereas the wash water applied to the drum by spray nozzles to be presently described, passes into the wash water chamber 1|. To effect a sharp separation of the filtrate and wash water, the ports 45 and 46 in the valve bodies or pockets 4| are madeV relatively narrow and are staggered, the ports 45 being formed on the trailing end of each pocket and the ports 46 being formed on the leading end. It is for this reason that as shown in Figure 6, the leading end of the leading shoe section 13 is slightly in advance of the opposed leading shoe section 62.

Briey, then, the valve construction associated with the interior of the drum comprises a plurality of peripherally aligned laterally ported valve bodies or pockets depending from and communicating with the filtrate compartments formed on the surface of the drum. These valve bodies are arranged to rotate through a fixed arcuate channel formed on one side by three identical shoe sections and on the other side first by a wash water chamber, then a by-pass shoe section, and finally by a plain shoe section. As each compartment emerges from the tank its contained filtrate passes through the leading port 46, and as each filtrate compartment approaches its zenith, the port 45 of its associated valve body or pocket 4| is closed by the trailing shoe section 62 and the wash water passing through the filtrate compartment passes through the port 46 into the wash Water chamber I|. Upon passing beyond the chamber ll, the by-pass in the shoe section l2 equalizes the pressure on the interior and exterior of the compartment as it approaches its point of discharge. Finally after each compartment is discharged of its cake, the trailing shoe sections close both ports 45 and 46 until the compartment is again submerged in the pulp, whereupon the filtrate is free to pass through these ports into the interior of the drum.

. As shown in Figures 1 and 6, the wash Water chamber communicates with the hollow shaft 53 through a pipe 8| connected therewith. The outer end of the shaft is connected to a pipe or hose 82 which in turn may be connected with a barometric leg or other suitable source of vacuum,

not illustrated.

For the purpose of angularly adjusting lxthe' shoes and wash water chamber, an adjusting lever open as shown in Figure 1 and its outer end is connected through a stuffing box 9| with a fitting 92 formed with a flange 93 by which this fitting may be connected, for example to a barometric leg or vacuum receiver, not shown.

Formed integral with the bearing 2| is a gear housing 94 containing a gear 95 in mesh with a bull gear 96 keyed to the trunnion 23. The gear 95 is keyed to a shaft 91 and the shaft 91 carries a pulley 98 driven through a belt 99 by an electric motor |0| supported on the tank frame or other suitable support.

Extending longitudinally within the fil-ter tank and held in spaced relation with the intake ports I6 by lugs |02 is a baille plate |03. The purpose of this baille plate is to prevent a dire'ct current of pulp towards the filter drum and to cause the pulp on entering the tank to pass downwardly below the baille and then upwardly in contact with the rising side of the drum. Bythis action the cake formed on the drum is prevented from sloughing off as it approaches the pulp level.

Disposed within the lter tank on the rising side of the drum is a longitudinally extending manifold |04 carrying a plurality of downwardly directed longitudinally spaced nozzles |05, through which water or other iiuid may be intro-f duced to augment the action of the baille plate |03.

Supported by the tank heads |3 and |4 on the rising side of the drum are brackets |06 formed with bearings |01. Journaled in these bearings are pins |08 to which are keyed depending straps |09 terminating in bearings Journaled in the bearingslll is a longitudinally extending shaft I2 on which is mounted a press roll I3 arranged to contact and compact the cake carried on the filter drum. Also keyed to the pins |08 are elbows ||4 (one at either end of the filter) to 5 are counterweights I6 by means of which the pressure of the roll ||3 against .the filter drum may be controlled.

Disposed above the rising side of the-drum and extending between the tank heads I3 and |4 are a plurality of wash Water manifolds each provided wth a plurality of longitudinally spaced spray nozzles IIB. 'Ihe outer ends of the nozzles are closed by a head l i9 and are formed with an inwardly cut orifice |26 so that they will produce a fan-shaped spray, and the nozzles are so spaced that the spray from one orifice will overlap the,

spray from its laterally adjacent orifice.

The cake formed on the filter drum after being subjected to a spray wash is discharged by means of a doctor provided on its lower side with a plurality of inwardly directed nozzles. The tip of the doctor is downwardly inclined and serves to direct a current of steam or other fluid passing through the nozzles, downwardly into the filtrate compartments immediately in advance of the sheet of cake o r solids to be discharged. Upon entering the filtrate compartment the steam creates an outward pressure which lifts the sheet away from the lter medium and allows it to pass outwardly over the surface of the doctor. To prevent the formation of a partial vacuum between the sheet 'and .the doctor the upper face of tudinally spaced ribs which hold the sheet out of strict surface contact with the face of the doctor.

As shown in Figures 7, 8, and 9, the doctor comprises a longitudinally extending plate |2| having a downwardly inclined lip I2|l and formed at either end with upstanding side members |22. Carried by these -side members at their outer ends and below the plate |2| is a longitudinally extending manifold |23 connected at each end to an upstanding intake pipe |24. The lip I2| 'of the plate |2| is formed with a vplurality of spaced notches |25 forlaccommodating the longitudinally spaced Acme bands 36 secured over the filter medium35. Disposed over the upper face of the plate |2| are a plurality of `ribs |26 for maintaining the cake out of strict surface contact with the plate |2|. Extending inwardly from the manifold |23 immediately below and parallel with the plate |2| are a plurality of longitudinally spaced nozzles |21, preferably terminating somewhat short of the lip |2|* but in alignment therewith. To prevent the steam jets from the nozzles |21 from taking ithe path of least resistance through the notches |25, the lower side of the plate |2| is provided with transverse bailles |28 on either side of each notch. In order to stiifen the plate |2I, these baffles have been made in the form of channels and are welded at their outer ends to the manifoldv |23 as well as over their entire length to the lower side of the plate I2 I. In side elevation, these channels are wedge shaped, tapering from their outer ends to their inner ends. Welded to each of the upstanding intake'pipes |24v in a vertical plane is a wing |29 formed at its outer end with a slot |3|. The doctor in its entirety is designed to be supported between the tank heads I3 and |4 by pins |32 carried by the tank heads and extending through the slots |3|, with the outer end of the plate |.2| resting on a downwardly anged seat |33 welded to the tank. I1', however, it is desired to adjust the inclination of the doctor, this can be done by means of a bolt |34 pivotally secured to one of the intake pipes |24 and passing through a lug |35 carried by the tank head. To swing the doctor away from its operative position and from its seat, it is vmerely necessary to move it upwardly and outwardly along the slot |3|.

Steam is preferably used as a discharging medium for the reason that some pulps such as paper pulp, carry an appreciable quantity of gums and resins which adhere to the filter medium and Acme bands and which can be readily dissolved by steam.

We claim:

1. A filter comprising: a rotary drum closed at both ends and arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed filtrate compartments formed on the surface of said drum and covered with a filter medium; peripherally aligned valve bodies communicating with said filtrate compartments and depending into and communicating with the interior of said drum; a port formed on each side of each valve body; a pair of stationary shoes disposed within said drum immediately adjacent the sides of said valve bodies and arranged to close said ports once during each revolution of said drum; a cake discharge device mounted adjacent the outer surface of said drum at a point opposite said shoes; and means for maintaining a diil'rential filtering pressure between the exterior and interior of said drum.

' at both ends and arranged to contact a body of 2,263,852 the doctor is provided with a p1urauty of iongi- 2. A filter comprising: a rotary drum arranged to contact a body of liquid to be ltered; a plurality of longitudinally disposed ltrate compartments formed on the surface of said drum and covered with a lter medium; peripherally aligned valve bodies depending from and communicating with said ltrate compartments, the opposed sides of said valve bodies defining a pair of opposed surfaces of revolution upon the rotation of said drum; a port formed on the leading edge of one side of each valve body and a port formed on the trailing edge of the other side of eachvalve body; a pair of stationary shoes disposed within said drum immediately adjacent the sides of said valve bodies and arranged to close their said ports once during each revolution of said drum; a cake discharge device mountedk adjacent the outer surface of said drum at a point opposite said shoes; and means for maintaining a differential filtering pressure between the exterior and interior of said drum.

3. A filter comprising: a rotary drum closed at both ends and arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed ltrate compartments formed on the surface of said drum and covered with a filter medium; peripherally aligned valve bodies communicating with said ltrate compartments and depending into and communicating with the interior of said drum; a port formed on each side of each valve body; a member extending through one end of said drum along its axis; a pair of spaced shoes carried by said member in peripheral alignment with and arranged to straddle said valve bodies so as Vto close each port therein once during each rotation of said drum; and means. for maintaining a differential filtering pressure between the exterior and lnterior of said drum.

4. A filter comprising: a rotary drum closed liquid to be filtered; a plurality of longitudinally disposed filtrate compartments formed on the surface of said drum and covered with a lter medium; peripherally aligned valve bodies communicating with said filtrate compartments and depending into and communicating with the interior of said drum; a port formed on one side of each valve body; a hollow shaft extending through one side of said drum; a shoe supported by said shaft in peripheral alignment with the ports in said valve bodies and arranged to close each of said ports once during each revolution of said drum; a conduit carried by and in communication with said shaft, the outer end of communicating with said filtrate compartments;

a port formed on one side of each valve body, said ports lying on a common surface of revolution; a stationary shoe disposed within said drum for concurrently subtending at least two of said ports, said shoe being located adjacent that portion of the drum from which the cake formed thereon is to be discharged; a by-pass associated with said shoe and arranged to establish communication between two of said subtended ports; and means for maintaining a differential pressure between the exterior and interior of said drum.

6. -A filter comprising: a rotary drum arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed filtrate compartments formed on the surface of said drum and covered with a lter medium; peripherally aligned valve bodies depending from and communicating with said filtrate compartments; a port formed on one side of each of said valve bodies, said ports lying on a common surface of revolution; a hollow shaft extending through one end of said drum; a stationary shoe supported by said shaft for concurrently subtending at least two of said ports,` said shoe being located adjacent that portion of the drum from which the cake formed thereon is to be discharged; a bypass associated with said shoe and arranged to establish communication between two of said subtended ports; a stationary chamber disposed within said drum adjacent the locus of said ports and in communication with said hollow shafts; and means for maintaining a differential pressure between the exterior and interior of said drum.

7. A filter comprising: a rotary drum arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed filtrate compartments formed on vthe surface of said drum and covered with a lter medium; peripherally aligned valve bodies depending from and communicating with said filtrate compartments; a port formed on one side of each valve body, said ports lying on va common surface of revolution; a stationary shoe disposed within said drum for concurrently subtending at least two of said ports, said shoe being located adjacent that portion of the drum from which the cake formed thereon is to be discharged; a by-pass associated with said shoe having an open channel arranged to establish communication between two of said subtended ports; means for blanking off any desired portion of said channel; and means for maintaining a differential pressure between the exterior and interior of said drum.

nions intermediate the ends thereof; a dummy drum head secured to the inner end of each of said trunnions; a filter drum secured to said outer and dummy drum heads, said drum and dummy drum heads defining a central filtrate `compartment; a plurality of longitudinally disposed filtrate cells formed on the surface of said drum and covered with a filter medium; peripherally aligned valve bodies depending from said filtrate cells and communicating with said central filtrate compartment; a port formed on each side of each valve body; a shaft bearing carried by each end of one of said trunnions; a shaft journaled in said shaft bearings; a pair of spaced shoes carried by said shaft in peripheral alignment with and arranged to straddle said valve bodies so as to close each port therein once during each revolution of said drum; and means for maintaining a differential filtering pressure between the exterior and interior of said drum.

11. A filter comprising: a slurry tank; aligned hollow trunnion bearings associated with each end of said tank; a trunnion journaled in each of said bearings and extending inwardly thereof; an outer drum head secured to each 'of said trunnions intermediate the ends thereof; a dummy drum head secured to the inner end of each of said trunnions; a filter drum secured t0 said outer and dummy drum heads, said drum and dummy drum heads defining a central filtrate compartment; a plurality of longitudinally disposed filtrate cells formed 0n the surface of said drum and covered with a filter medium; peripherally aligned valve bodies depending from said filtrate cells and communicating with said filtrate compartment; a port formed on each side of each valve body; a shaft bearing carried by each end of one of said trun- 8. In a continuous filter including a filter cell arranged to travel in a closed path, a doctor comprising: a plate mounted adjacent and trans-'f versely to the path of travel of said filter cell;

a manifold extending longitudinally along the lower face of said plate; and a plurality of spaced nozzles connected to said manifold and aligned with the lower forward surface of said plate so connected to said manifold and aligned with the lower forward surface of said plate so that jets from said nozzles will be deflected downwardly from said lower forward surface; and a plurality of spaced ribs formed on the upper surface of said plate and extending transversely thereof.

10. A filter comprising: a slurry tank; aligned hollow trunnion bearings associated with each end of said tank; a trunnion journaled in each of said bearings and extending inwardly thereof; an outer drum head secured to each of said trunnions; a, hollow shaft journaled in said shaft bearings; a pair of spaced shoes fixed to said shaft in peripheral alignment with and arranged to straddle said valve bodies so as to close each port therein once during each revolution of said drum; a conduit carried by and communicating at one end with said hollow shaft and having its other end in peripheral alignmentl with the ports in said valve bodies at a point above the normal liquor level in said filtrate compartment so' as to establish communication between each of said ports and said hollow shaft at this point once during each revolution of said drum; and means for maintaining a differential filtering pressure between the exterior and interior of said drum.

12. A filter comprising: a rotary drum closed at both ends and arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed filtrate compartments formed on the surface of said drum and covered with a filter medium; peripherally aligned valve bodies depending from and communicating with said filtrate compartments; a port formed on one side of each valve body for permitting filtrate to pass from said filtrate compartments into said drum, said ports lying on a common surface of revolution; a stationary shoe disposed within said drum at a point above the normal filtrate level therein for concurrently subtending at least two of said ports; a by-pass associated with said shoe and arranged to establish communication between two of said subtended ports; and means for maintaining a differential pressure between the exterior and interior of said drum.

13. A filter comprising: a rotary drum arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed lltrate compartments formed on the surface of said drum and covered with a filter medium; peripherally aligned valve bodies depending from `and communicating with said filtrate compartments; a port formed on one side of each valve body, said ports lying on a common surface of revolution; a stationary shoe disposed within said drum for concurrently subtending at least two of said ports; a by-pass associated with said shoe and arranged to .establish communication between two of said subtended ports; and means for maintaining a differential pressure between the exterior and interior of said drum.

14. A filter comprising: a rotary drum arranged to contact a body of liquid to be filtered; a plurality of longitudinally disposed filtrate compartments formed on the surface of said drum and covered with a fllter medium; peripherally aligned valve bodies depending from and communicating with said ltrate compartments; a port formed on one side of each of said valve bodies, said ports lying on a common surface of revolution; a hollow shaft extending through one end of said drum; a stationary shoe supported by said shaft for concurrently subtending at least two of said ports; a by-pass associated with said shoe and arranged to establish communication between two of said subtended porta: a stationary chamber disposed within said drum adjacent the locus of vsaid ports and in communication with said hollow shaft: and means for maintaining a differential pressure between the exterior and interior of said druln.

15. A filter comprising: `a rotary drum arranged to contact a body of liquid to beltered; a plurality of longitudinally ydisposed filtrate compartments formed on the surface of said drum and covered with a filter medium; peripherally aligned valve bodies depending from and communicating with said filtrate compartments; a port formed on one side oi each valveybody, said ports lying on a common surface of revolution; ia stationary shoe disposed within said drum for concurrently subtending at least two of said ports; a by-pass associated with said shoe having an open channel arranged to establish communication between two of said subtended ports; means for blanking on any desired portion of said channel; and means for maintaining a diierential pressure vbetween the exterior and interior voi' said drum.

EDWIN- L.A OLIVER. EDWARD D. FLYNN. Y ORVILLE B. AOKERLY, JR. 

